Congenital heart disease (CHD) is the most common birth defect and a leading cause of morbidity and mortality in children. While the genetic cause of some types of CHD has been identified, the basis for most forms of sporadic heart disease remains unknown. Among the various forms of CHD, hypoplastic left heart syndrome (HLHS) appears to have a particularly high degree of heritability and is often associated with sub- clinical aortic valve disease in first-degree relatives. One of the major co-morbidities associated with CHD, and particularly HLHS, involves poor neurodevelopmental outcomes. While the etiology remains unclear, there is evidence to support a patient-intrinsic component to the neurologic outcome, which often is subtle and presents at school age. There is also an increased incidence of neuro-anatomic abnormalities in patients with HLHS, particularly agenesis or hypoplasia of the corpus callosum, but the genetic basis for this is unclear and it is unknown if poor neurologic outcomes are related to anatomic anomalies. In the Main Project of this proposal, we aim to test the hypothesis that HLHS arises from genetic variants with relatively strong effects and that in at least a subset of cases, the same genetic variants disrupt both aortic valve and neuronal development. By evaluating the combination of HLHS and neuro-anatomic anomalies as a unique syndrome, we may identify a common genetic etiology within this more homogeneous subset even when index cases are unrelated. In the associated Neurodevelopmental Project, we will test the hypothesis that adverse neurodevelopmental outcomes in HLHS have a genetic component and may independently be associated with neuro-anatomic defects. To achieve these goals, the we plan to do the following: 1) Enroll subjects with HLHS and collect DNA, phenotypic data, determine sub-clinical neurologic anatomies by MRI, and assess their neurodevelopmental outcomes; 2) Determine genetic variants through next-generation sequencing that are associated with HLHS with or without co-existence of anatomic neurodevelopmental anomalies and/or abnormal neurodevelopmental outcomes; 3) Experimentally determine if genetic variants that segregate with HLHS or with poor neurodevelopmental outcomes are function-altering and contribute to pathologies associated with disease by using human induced pluripotent stem (iPS) cells and state-of-the-art genome editing techniques. These aims will be pursued through two integrated projects that study the same cohort of patients and leverage data from each to discover genetic causes of HLHS and the associated poor neurodevelopmental outcomes. We have assembled a team of cardiologists, neurodevelopmental experts, geneticists, and computational biologists at UCSF, Gladstone, and Stanford to accomplish these goals, which could only be accomplished in the context of a national consortium.